Solar Absorber for Concentrated Solar Power Generation

ABSTRACT

A solar absorber for a concentrated solar power (CSP) system includes a cylindrically shaped blackbody cavity receiver fused to a cylindrically shaped heat exchanger shell covering the receiver to form a monolithic cavity receiver and heat exchanger. An exterior surface of the cavity receiver has grooves embedded to create a duct spiraling about the exterior of the cavity receiver so that the duct forms tubes of a tube-style heat exchanger when covered by the heat exchanger shell. An interior diameter of the cavity receiver is greater than or equal to a diameter of an aperture of the cavity receiver, and a longitudinal interior depth of the cavity is greater than or equal to twice the interior diameter of the cavity receiver. The monolithic solar absorber is preferably composed of a ceramic material such as silicon carbide having emissivity greater than 0.9.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 13/789214 filed Mar. 7, 2013, which claims priority from U.S. Provisional Patent Application 61/607747 filed Mar. 7,2012, both of which are incorporated herein by reference.

STATEMENT OF GOVERNMENT SPONSORED SUPPORT

This invention was made with Government support under contract SU-83603201 awarded by Environmental Protection Agency. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to solar energy devices. More particularly, the invention relates to solar absorbers for concentrated solar power (CSP) systems.

BACKGROUND OF THE INVENTION

Concentrated solar power (CSP) technology seeks to replace fossil fuels in mechanical compression cycles for power generation. Two common types of CSP systems are dish systems and heliostat field systems. A conventional CSP dish system is described, for example, in U.S. Pat. No. 4,335,578 to Osborn et al., which is incorporated herein by reference. The system includes a large parabolic dish reflector mounted on a base with a pivoting mechanism. The dish reflects solar rays to a solar absorber positioned at its focal point. The solar absorber has a cylindrical cavity that collects the focused solar energy. The absorber is a double-walled vessel with a secondary working fluid in sealed annular spaces between the walls. Solar energy vaporizes the secondary working fluid which then passes from the absorber to a heat exchanger where it transfers heat to a primary working fluid. A Stirling cycle engine is then used to convert the heat in the primary working fluid into electrical energy. Osborn uses a phase change heat transfer fluid in conjunction with an auxiliary heat exchanger. Such phase change heat exchangers thus use a second step in the heat transfer process that adds complexity, inefficiency and expense to the design.

These types of CSP systems require large areas to achieve sufficient conversion efficiency for the cost of the system. In addition, the conversion efficiencies which have been achieved with such systems are limited by the materials and manufacturing processes used. For example, the current solar absorbers in the art use metal components in one form or another, which limits the overall performance that can be achieved. Common high temperature metals will begin to fail around 1300 K, and these metals have a low thermal conductivity. Since metal has high reflectivity, the absorbers are often made large to increase their cavity surface area, and their cavity surface is often coated with a radiation absorbent layer.

US Pat. Appl. Pub. 2013/0025586 to Hack describes an improved solar absorber module design with air-flow channels passing through it. Hack uses a volumetric style heat exchanger, which has a non-uniform temperature field and produces inconsistencies such as transient hot spots, and also imposes heat losses on the working fluid.

There is thus a need in the art for improved solar absorbers.

SUMMARY OF THE INVENTION

According to one aspect, the invention provides a solar absorber for a concentrated solar power (CSP) system. The solar absorber includes a ceramic blackbody cavity receiver and a ceramic heat exchanger shell covering the receiver, both of which may be cylindrical or have a polygonal cross-section. The cavity receiver and heat exchanger shell are fused to form a monolithic cavity receiver and heat exchanger. An exterior surface of the cavity receiver has grooves embedded to create a duct spiraling about the exterior of the cavity receiver so that the duct forms tubes of a tube-style heat exchanger when covered by the heat exchanger shell. An interior diameter of the cavity receiver is greater than or equal to a diameter of an aperture of the cavity receiver, and a longitudinal interior depth of the cavity is greater than or equal to twice the interior diameter of the cavity receiver. The cavity receiver is composed of a first monolithic ceramic material having emissivity greater than 0.9 and the heat exchanger shell is composed of a second monolithic ceramic material having emissivity greater than 0.9. The first and second ceramic materials are not necessarily distinct types of materials, but may be.

In a preferred embodiment, the first monolithic ceramic material is silicon carbide and the interior diameter of the cavity receiver is equal to the diameter of the aperture of the cavity receiver. Alternatively, the first monolithic ceramic material may be silicon nitride. Preferably, the duct has a rectangular cross-section. In some embodiments, instead of circular cross-sections resulting in cylindrical shapes, the blackbody cavity receiver and heat exchanger shell covering the receiver have square, rectangular, or hexagonal cross-sections. These cross-sections are preferable in embodiments including an array of solar absorbers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a solar absorber with a blackbody cavity absorbing solar radiation and heating air as it circulates around the cavity, according to an embodiment of the invention.

FIG. 2 shows a detail view of the of a receiver having air passage ducts integrated onto its surface, according to an embodiment of the invention.

FIG. 3 shows an embodiment of the invention where the final section of the heat exchanger is designed as a nozzle for expelling hot air.

FIG. 4 shows a panel array of absorbers having a square cross-sectional shape, according to an embodiment of the invention.

FIG. 5 shows a panel array of absorbers having a hexagonal cross-sectional shape, according to an embodiment of the invention.

DETAILED DESCRIPTION

In one embodiment, a scalable, modular, low cost, high efficiency solar absorber is provided for a concentrated solar power system such as a thermal dish-Brayton system or heliostat field system. It has reduced production costs and creates a viable small-scale solar power system for home or neighborhood use by achieving better conversion efficiency through its capability to achieve much higher operating temperatures than current receivers.

FIG. 1 shows a schematic drawing of a solar absorber according to an embodiment of the invention. The absorber includes a blackbody radiation receiver 100 and heat exchanger shell 102. An air passage entrance 104 through the shell serves as a cool air inlet and air passage exit 106 serves as hot air outlet. Also shown is the helical air passage duct 108 around the solar cavity receiver 100. This embodiment of the invention uses a cylindrical shaped blackbody cavity 112 of the receiver 100 to absorb incident solar radiation 110 at a temperature of above 1600 K or, more preferably, above 1700 K, which are much higher temperatures than existing receivers which absorb radiation at no more than 1250 K. This higher temperature is made possible by fabricating the absorber entirely using a ceramic such as silicon carbide, i.e., the absorber has no metal components. The fabricated cylindrical part boasts high absorption and thermal conductivity at a low fabrication cost. The advantage of the high absorption allows for the cylindrical design and a more compact absorber than current designs, which require much larger surface areas.

The high thermal conductivity allows an integrated heat exchanger to heat a working fluid to a design temperature of above 1400 K or, more preferably, above 1500 K. This high working fluid temperature will increase the conversion efficiency by 20% over comparable systems. The increased working temperature also enables better waste heat recovery by increasing the temperature of the Brayton Cycle exhaust. The integrated heat exchanger also achieves the desired fluid temperature with very little loss in pressure from entrance to exit.

FIG. 2 shows a detailed view of the outer surface of the receiver 100 without the heat exchanger shell, showing square cross-sectional grooves 108 embedded in the surface of the receiver between ridges 114, according to one embodiment of the invention. The grooves allow air to travel around the receiver once the shell 102 is covering the receiver, heating the air effectively without large losses in air pressure.

In a preferred embodiment, the duct cross-sectional shape is rectangular or, more preferably, square. The size of the square is chosen to accommodate necessary length of channels on the surface of the receiver with small pumping power. Other cross-sectional shapes are possible as well. A semi-circular, triangular or trapezoidal cross-section can be used, although a square duct is generally preferred because it offers the best balance between head losses and heat transfer to the fluid. In one embodiment, the ducts 108 have a depth of 10 mm and a width of 10 mm, with adjacent ducts separated by 4 mm wide ridges 114. In operation, a compressor pumps air into duct 108, causing it to flow through the duct 108 (e.g., at a mass flow rate of 9.3 g/s). As the air flows through the helical path of the duct around the receiver, it heats to 1500 K, achieving a Reynolds number of 4850 for maximum heat transfer per unit length. A total duct length of 3.09 m provides an effectiveness of 0.82. A pressure drop of 40 Pa is calculated for the length of the duct. After heating up inside the duct, the air flows out through a turbine. The turbine, in turn, drives both the compressor and a generator for power generation. For example, in gas turbine generators the compressor, turbine, and alternator are all on a common shaft. When the turbine spins, the compressor and alternator both spin in unity.

The ducts in the embodiment shown are portions of a single duct that wraps around the circumference of the receiver to form a helix. In other embodiments, there may be multiple interleaved helical ducts, each with its own input and output at opposite longitudinal ends of the receiver.

FIG. 3 illustrates an absorber according to an embodiment of the invention where the final section of the heat exchanger shell 302 is designed as a nozzle 300 in the same way that the first stage stator would be for a gas turbine. The exit of the air flow duct is routed to a central longitudinal axis of the device from the circumference at the hot end of the receiver 304. This nozzle 300 allows integration with a power turbine achieving fewer parts and a more compact package, allowing the omission of the first stage stator making the overall design lighter, cheaper and less expensive.

An important advantage of absorbers according to the present invention is the relative ease of manufacture. In contrast with absorbers of the present invention, a conventional absorber has to be manufactured in a multistage processes. After the absorber is built out of metals, which have low emissivity and need more complex geometry, an anti-reflection coating is applied and heat exchangers must be integrated. Absorbers according to the present invention, on the other hand, avoids all of these complications by manufacturing the receiver in one casting-in-mold process. The casting process is a more rapid and cheaper manufacturing process than machining blanks To do this, minimum draft angles and non-overlapping regions are used for the pieces to pull out of the molds as well as consideration for mold features such as channels for material to flow into. Standard mold design techniques for casting ceramic pieces may be used.

The receiver and shell are both composed of ceramic materials that are cast molded and then fused together to form a single, monolithic ceramic absorber device. Preferably, the ceramic materials have an emissivity higher than 0.9. For example, in a preferred embodiment the receiver and shell ceramic materials may be silicon carbide. In an embodiment where a silicon carbide receiver has a cylindrical cavity, an interior diameter of the cavity is preferably equal to a diameter of an aperture of the cavity receiver, and a longitudinal interior depth of the cavity is equal to or greater than twice an interior diameter of the cavity, as illustrated in FIG. 3. These dimensions are important due to the very high emissivity of silicon carbide.

Because silicon carbide is in some respects difficult to work with, other ceramic materials of lesser emissivity may be used. In such embodiments, it is preferred that the aperture of the receiver be less than an interior diameter of the cavity, e.g., by integrating an aperture ring into one of the sintered parts. It is also preferred that the ceramic materials of the receiver and shell are compatible with being sintered together or to one another. In the case of using a material different than silicon carbide, the duct dimensions and cavity dimensions would be modified to optimize for the specific materials' properties. For example, the duct dimensions may be optimized based on basic inviscid fluid equations. The maximum heat transfer per unit length of duct is sought, which is a direct function of the duct cross-sectional area.

Cavity dimensions are based on radiative heat transfer equations. Specifically, the effective emissivity of the cavity may be calculated by evaluating the view factor of the cavity aperture to the inner walls of the cavity. Overall effective emissivity is a function of the emissivity of the materials' properties, so materials with lower emissivity require a greater interior surface area to aperture ratio.

In an alternate embodiment, the ceramic materials of the receiver and shell are both silicon nitride. While this material is easier to work with than silicon carbide, it has lower emissivity and thermal conductivity than silicon carbide. A limiting factor for which ceramic materials may be employed is how readily they cofire, or sinter, to one another. Certain metal-metal oxide ceramics are bonded together using an interstitial layer of refractory metal and is also a potentially viable process, but it is extremely expensive. Common metals are even able to be sintered, but they are often coated with an anti-reflecting coating and this is expensive. The advantage of silicon carbide is that the high emissivity and high thermal conductivity allow efficient radiation absorption and heat transfer, and this results in a smaller overall device.

In preferred embodiments, the cross-sectional shape of the receiver cavity (as well as the shell) is a circle (i.e., the cavity is a cylinder). In alternate embodiments, the cross-sectional shape of the cavity is a polygon, or more preferably a regular polygon. In general, the polygonal cross-section is preferably such that multiple absorbers may be packed together with no gaps between them to create a panel of absorbers. For example, square absorbers 400 may be stacked next to one another without any lost area between them to form a panel 402, as shown in FIG. 4. Similarly, hexagonal absorbers 500 may be stacked next to one another without any lost area between them to form a panel 502, as shown in FIG. 5. Such a panel includes multiple receivers stacked closely next to one another with their apertures in the same plane. As a result, the absorber is easily scalable to make a larger scale absorber. The individual polygonal cross-section absorbers in such a panel are manufactured the same way as cylindrical absorbers. The absorbers are then stacked or mounted one atop one another to form the panel array. This type of absorber array has useful application in large heliostat array systems.

The absorbers of the present invention may be mass produced and have primary application to the commercialization of small-scale solar energy production for the end user, where they may replace existing absorbers in such systems. For example, a 17 m² reflector dish may be used to focus solar energy into a 72 mm aperture absorber of the present invention. Assuming 900 W/m² solar energy density and 90% mirror reflectivity, the absorber will output air at 1500 K. The absorber output may be coupled to a microturbine that in turn is paired with a generator to produce 2.5 kW of electricity. The 1100 K turbine exhaust air represents 5 kW of useful heat that may be used for other purposes, such as space heating, hot water heating, and absorption chilling. The high emissivity of silicon carbide allows the absorbers to be highly absorptive in a compact size and without any absorptive coatings. Silicon carbide is also operable to temperatures in excess of 2000 K, which is beyond the requirements of this system. The 1500 K fluid target temperature of this system will boast a 20% increase in efficiency over similar systems.

In alternate embodiments, the absorber can be scaled to various sizes. For example, absorber dimensions may range from 50 mm aperture diameter and 100 mm cavity depth up to 560 mm aperture diameter and 1120 mm cavity depth, corresponding respectively to 2.5 kW and 50 kW power generation for utility scale power production.

Although the primary intended application of the absorbers of the present invention is to solar power production using a Brayton power cycle, the high temperature air from these absorbers have other applications as well. For instance, the air can be used to heat water for process applications, or to superheat steam to supplement fossil fuels in a coal fired power plant.

Although the absorbers in preferred embodiments of the present invention are designed to function with air as the working fluid, the chemical stability of silicon carbide makes this absorber a candidate for thermochemical reactions for the solar production of synthetic fuels, or reformation of light hydrocarbons into heavier ones. Absorption chillers or water heaters may also take advantage of the waste heat from any of the aforementioned methods of heat utilization. 

1. A solar absorber for a concentrated solar power (CSP) system, the solar absorber comprising a ceramic blackbody cavity receiver and a ceramic heat exchanger shell fused to each other to form a monolithic cavity receiver and heat exchanger; wherein an exterior surface of the cavity receiver has grooves embedded to create a duct spiraling about the exterior of the cavity receiver, wherein the heat exchanger shell covers the receiver such that the duct forms tubes of a tube-style heat exchanger; wherein an interior diameter of the cavity receiver is greater than or equal to a diameter of an aperture of the cavity receiver, wherein a longitudinal interior depth of the cavity is greater than or equal to twice the interior diameter of the cavity receiver.
 2. The solar absorber of claim 1 wherein the cavity receiver and the heat exchanger are composed of a ceramic material having emissivity greater than 0.9.
 3. The solar absorber of claim 1 wherein the cavity receiver and the heat exchanger are composed of silicon carbide.
 4. The solar absorber of claim 3 wherein the interior diameter of the cavity receiver is equal to the diameter of the aperture of the cavity receiver.
 5. The solar absorber of claim 1 wherein the cavity receiver and the heat exchanger are composed of silicon nitride.
 6. The solar absorber of claim 1 wherein the duct has a rectangular cross-section.
 7. The solar absorber of claim 1 wherein the blackbody cavity receiver and the heat exchanger shell have a cross-sectional shape selected from the group consisting of a rectangle, square, hexagon, and circle.
 8. The solar absorber of claim 1 wherein the heat exchanger shell and blackbody cavity receiver both are cylindrically shaped. 